ASPIRE

Atmosphere-Induced Short Period Variations of Earth Rotation

Thorough investigation of Earth rotation irregularities plays a key role in advancing the
accuracy of space geodetic products, which have become truly indispensable to global positioning
tasks. Moreover, dissecting the observed variability of the rotation axis in terms of its
geophysical origin promotes insight into Earth system dynamics. A considerable part of the
modern-day interest in those effects pertains to short periods, encompassing also a minute but
measurable influence of diurnal and semidiurnal atmospheric tides on polar
motion and changes in length of day (LOD). The physical character of
this forcing is known to be twofold — the atmosphere exchanges angular momentum with the
solid Earth both directly through boundary layer stresses and indirectly through the
oceans — but existing quantifications of the elicited rotational perturbations are still
at discord and thus a source of uncertainty in the analysis and interpretation of space geodetic data.

Illustration of angular momentum conservation (+/- arrows)
in an Earth-fluid layer system as a semidiurnal pressure wave occurs in the atmosphere.
The alternative torque method to model the effect of geophysical fluid dynamics on Earth
rotation uses globally integrated forces, such as the "push and pull" of pressure on
mountain ranges and the tangential friction drag exerted by surface winds.

Project ASPIRE is conceived as a comprehensive treatise on the
nature and scope of atmosphere-induced (semi)diurnal polar motion signals and changes in LOD
from the viewpoint of geophysical modeling. The central idea is to determine 3-hourly estimates
of both atmospheric angular momentum (AAM) and Earth-atmosphere torques (see figure) from a carefully
selected pool of meteorological analysis-forecast data of three state-of-the-art atmospheric
assimilation systems:

ERA-Interim, the current reanalysis of the European Centre for
Medium-Range Weather Forecasts,

MERRA (Modern Era-Retrospective Analysis for Research and
Applications) produced by NASA's GMAO (Global Modeling and Assimilation Office), and

The National Centers for Environmental Prediction (NCEP) Climate
Forecast System Reanalysis (CFSR).

A mutual comparison of individual (semi)diurnal excitation quantities (pressure and wind
AAM on the one side, ellipsoidal, mountain, and friction torques on the other side) among all
models indicates their reliability for short period Earth rotation studies. This inquiry is
augmented by an in-depth analysis of the required balance between the time derivative of
AAM and the total atmospheric torque. These investigations form the foundation for the
project's goal to deduce credible estimates of the associated excitation effects and
eventually advance the agreement between modeled and observed Earth rotation variations.

Being a joint project together with Section 1.3 Earth System Modelling of
Geoforschungszentrum Potsdam, the second area of operation within ASPIRE addresses the
dynamic ocean response to atmospheric tides using the global numerical Ocean
Model for Circulation and Tides (OMCT, Dobslaw and Thomas, 2005). Experiments with
regionally refined grids will provide the foundation on which oceanic angular momentum at
3-hourly intervals and the corresponding torques will be intercompared. The
combined atmosphere-ocean excitation values will be assessed on the evidence of Earth rotation
solutions from geodetic observing systems.